mybatis源码学习(三):MappedStatement的解析过程
我们之前介绍过MappedStatement表示的是XML中的一个SQL。类当中的很多字段都是SQL中对应的属性。我们先来了解一下这个类的属性:
public final class MappedStatement { private String resource; private Configuration configuration; //sql的ID private String id; //尝试影响驱动程序每次批量返回的结果行数和这个设置值相等 private Integer fetchSize; //SQL超时时间 private Integer timeout; //Statement的类型,STATEMENT/PREPARE/CALLABLE private StatementType statementType; //结果集类型,FORWARD_ONLY/SCROLL_SENSITIVE/SCROLL_INSENSITIVE private ResultSetType resultSetType; //表示解析出来的SQL private SqlSource sqlSource; //缓存 private Cache cache; //已废弃 private ParameterMap parameterMap; //对应的ResultMap private List<ResultMap> resultMaps; private boolean flushCacheRequired; private boolean useCache; private boolean resultOrdered; //SQL类型,INSERT/SELECT/DELETE private SqlCommandType sqlCommandType; //和SELECTKEY标签有关 private KeyGenerator keyGenerator; private String[] keyProperties; private String[] keyColumns; private boolean hasNestedResultMaps; //数据库ID,用来区分不同环境 private String databaseId; private Log statementLog; private LanguageDriver lang; //多结果集时 private String[] resultSets; MappedStatement() { // constructor disabled } ... }
对一些重要的字段我都增加了备注,方便理解。其中真正表示SQL的字段是SqlSource这个对象。
SqlSource接口很简单,只有一个getBound方法:
public interface SqlSource { BoundSql getBoundSql(Object parameterObject); }
它有很多实现,需要我们重点关注的是StaticSqlSource,RawSqlSource和DynamicSqlSource。在正式学习他们前,我们先了解一下Mybatis动态SQL和静态SQL的区别。
动态SQL表示这个SQL节点中含有${}或是其他动态的标签(比如,if,trim,foreach,choose,bind节点等),需要在运行时根据传入的条件才能确定SQL,因此对于动态SQL的MappedStatement的解析过程应该是在运行时。
而静态SQL是不含以上这个节点的SQL,能直接解析得到含有占位符形式的SQL语句,而不需要根据传入的条件确定SQL,因此可以在加载时就完成解析。所在在执行效率上要高于动态SQL。
而DynamicSqlSource和RawSqlSource就分别对应了动态SQL和静态SQL,它们都封装了StaticSqlSource。
我们先从简单的入手,了解静态SQL的解析过程。
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="org.apache.ibatis.domain.blog.mappers.AuthorMapper"> <select id="selectAllAuthors" resultType="org.apache.ibatis.domain.blog.Author"> select * from author </select> </mapper>
这是我们要解析的XML文件,mapper节点下只有一个select节点。
public class XmlMapperBuilderTest { @Test public void shouldSuccessfullyLoadXMLMapperFile() throws Exception { Configuration configuration = new Configuration(); String resource = "org/apache/ibatis/builder/AuthorMapper.xml"; InputStream inputStream = Resources.getResourceAsStream(resource); XMLMapperBuilder builder = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); builder.parse(); inputStream.close(); } }
这是我们测试解析过程的代码。我们可以看到解析是由XMLMapperBuilder开始的。我们先了解一下它的字段:
public class XMLMapperBuilder extends BaseBuilder { //用来解析XML private final XPathParser parser; //再解析完成后,用解析所得的属性来帮助创建各个对象 private final MapperBuilderAssistant builderAssistant; //保存SQL节点 private final Map<String, XNode> sqlFragments; //... }
它还从父类中继承了configuration(配置对象),typeAliasRegistry(类型别名注册器)和typeHandlerRegistry(类型处理器注册器)。
接下来看一下它的parse方法:
public void parse() { //判断是否已经加载过资源 if (!configuration.isResourceLoaded(resource)) { //从mapper根节点开始解析 configurationElement(parser.evalNode("/mapper")); //将该资源添加到为已经加载过的缓存中 configuration.addLoadedResource(resource); //将解析的SQL和接口中的方法绑定 bindMapperForNamespace(); } //对一些未完成解析的节点再解析 parsePendingResultMaps(); parsePendingCacheRefs(); parsePendingStatements(); }
主要的解析过程在configurationElement中:
private void configurationElement(XNode context) { try { //解析mapper的namespace属性,并设置 String namespace = context.getStringAttribute("namespace"); if (namespace == null || namespace.equals("")) { throw new BuilderException("Mapper's namespace cannot be empty"); } builderAssistant.setCurrentNamespace(namespace); //解析<cache-ref>节点,它有一个namespace属性,表示引用该命名空间下的缓存 cacheRefElement(context.evalNode("cache-ref")); //解析<cache>节点,可以设置缓存类型和属性,或是指定自定义的缓存 cacheElement(context.evalNode("cache")); //已废弃,不再使用 parameterMapElement(context.evalNodes("/mapper/parameterMap")); //解析resultMap节点 resultMapElements(context.evalNodes("/mapper/resultMap")); //解析<SQL>节点,SQL节点可以使一些SQL片段被复用 sqlElement(context.evalNodes("/mapper/sql")); //解析SQL语句(select|insert|update|delete节点) buildStatementFromContext(context.evalNodes("select|insert|update|delete")); } catch (Exception e) { throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e); } }
我们关注SQL语句的解析过程,上述buildStatementFromContext(List<XNode>)方法会增加dateBaseId的参数,然后调用另一个重载方法:
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { //遍历XNode节点 for (XNode context : list) { //为每个节点创建XMLStatementBuilder对象, final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId); try { //解析Node statementParser.parseStatementNode(); } catch (IncompleteElementException e) { //对不能完全解析的节点添加到incompleteStatement,在parsePendingStatements方法中再解析 configuration.addIncompleteStatement(statementParser); } } }
先看看XMLStatementBuilder对象:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
public class XMLStatementBuilder extends BaseBuilder { private final MapperBuilderAssistant builderAssistant; private final XNode context; private final String requiredDatabaseId; // ... }
含有的字段相对简单,不再具体解释。直接看parseStatementNode方法:
public void parseStatementNode() { //获取id String id = context.getStringAttribute("id"); String databaseId = context.getStringAttribute("databaseId"); //验证databaseId是否匹配 if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { return; } Integer fetchSize = context.getIntAttribute("fetchSize"); Integer timeout = context.getIntAttribute("timeout"); //已废弃 String parameterMap = context.getStringAttribute("parameterMap"); //参数类型;将会传入这条语句的参数类的完全限定名或别名。这个属性是可选的,因为 MyBatis 可以通过 TypeHandler 推断出具体传入语句的参数,默认值为 unset。 String parameterType = context.getStringAttribute("parameterType"); Class<?> parameterTypeClass = resolveClass(parameterType); //结果类型;外部 resultMap 的命名引用。 String resultMap = context.getStringAttribute("resultMap"); //结果类型;表示从这条语句中返回的期望类型的类的完全限定名或别名。注意如果是集合情形,那应该是集合可以包含的类型,而不能是集合本身。不能和resultMap同时使用。 String resultType = context.getStringAttribute("resultType"); String lang = context.getStringAttribute("lang"); LanguageDriver langDriver = getLanguageDriver(lang); Class<?> resultTypeClass = resolveClass(resultType); //结果集类型;FORWARD_ONLY,SCROLL_SENSITIVE 或 SCROLL_INSENSITIVE 中的一个,默认值为 unset (依赖驱动)。 String resultSetType = context.getStringAttribute("resultSetType"); //STATEMENT,PREPARED 或 CALLABLE 的一个。这会让 MyBatis 分别使用 Statement,PreparedStatement 或 CallableStatement,默认值:PREPARED。 StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString())); ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType); String nodeName = context.getNode().getNodeName(); //SQLCommand类型 SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH)); boolean isSelect = sqlCommandType == SqlCommandType.SELECT; //flushCache;在执行语句时表示是否刷新缓存 boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect); //是否对该语句进行二级缓存;默认值:对 select 元素为 true。 boolean useCache = context.getBooleanAttribute("useCache", isSelect); //根嵌套结果相关 boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false); //引入SQL片段 // Include Fragments before parsing XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant); includeParser.applyIncludes(context.getNode()); // Parse selectKey after includes and remove them. //处理selectKey processSelectKeyNodes(id, parameterTypeClass, langDriver); // Parse the SQL (pre: <selectKey> and <include> were parsed and removed) SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass); // String resultSets = context.getStringAttribute("resultSets"); String keyProperty = context.getStringAttribute("keyProperty"); String keyColumn = context.getStringAttribute("keyColumn"); //设置主键自增的方式 KeyGenerator keyGenerator; String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX; keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true); if (configuration.hasKeyGenerator(keyStatementId)) { keyGenerator = configuration.getKeyGenerator(keyStatementId); } else { keyGenerator = context.getBooleanAttribute("useGeneratedKeys", configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType)) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE; } //通过buildAssistant将解析得到的参数设置构造成MappedStatement对象 builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType, fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass, resultSetTypeEnum, flushCache, useCache, resultOrdered, keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets); }
将解析得到参数通过BuilderAssistant.addMappedStatement方法,解析得到MappedStatement对象。
上面已经说过sqlsource表示的一个SQL语句,因此我们关注langDriver.createSqlSource这个方法。看XMLLanguageDriver这个实现。
@Override public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) { XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType); return builder.parseScriptNode(); }
可以看到他将创建sqlsource的工作交给了XMLScrpitBuilder(又一个创建者模式的应用)。来看parseScriptNode方法:
public SqlSource parseScriptNode() { //解析SQL语句节点,创建MixedSqlNode对象 MixedSqlNode rootSqlNode = parseDynamicTags(context); SqlSource sqlSource = null; //根据是否是动态的语句,创建DynamicSqlSource或是RawSqlSource对象,并返回 if (isDynamic) { sqlSource = new DynamicSqlSource(configuration, rootSqlNode); } else { sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType); } return sqlSource; }
MixedSqlNode是SqlNode的一个实现,包含了各个子节点,用来遍历输出子节点。SqlNode还有很多不同的实现,分别对应不同的节点类型。对应关系如下:
SqlNode实现 | 对应SQL语句中的类型 |
TextSqlNode | ${} |
IfSqlNode | If节点 |
TrimSqlNode/WhereSqlNode/SetSqlNode | Trim/Where/Set节点 |
Foreach节点 | foreach标签 |
ChooseSqlNode节点 | choose/when/otherwhise节点 |
ValDeclSqlNode节点 | bind节点 |
StaticTextSqlNode | 不含上述节点 |
除了StaticTextSqlNode节点外,其余对应的都是动态语句。
因此我们本文的关注点在StaticTextSqlNode。
让我们对应文初sql语句的解析来看一下parseDynamicTags方法,为了便于理解,我将在右边注释出每一步的结果
protected MixedSqlNode parseDynamicTags(XNode node) {// node是我们要解析的SQL语句: <select resultType="org.apache.ibatis.domain.blog.Author" id="selectAllAuthors">select * from author</select> List<SqlNode> contents = new ArrayList<SqlNode>(); //获取SQL下面的子节点 NodeList children = node.getNode().getChildNodes();//这里的children只有一个节点; //遍历子节点,解析成对应的sqlNode类型,并添加到contents中 for (int i = 0; i < children.getLength(); i++) { XNode child = node.newXNode(children.item(i));//第一个child节点就是SQL中的文本数据:select * from author //如果是文本节点,则先解析成TextSqlNode对象 if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) { //获取文本信息 String data = child.getStringBody("");//data:select * from author //创建TextSqlNode对象 TextSqlNode textSqlNode = new TextSqlNode(data); //判断是否是动态Sql,其过程会调用GenericTokenParser判断文本中是否含有"${"字符 if (textSqlNode.isDynamic()) {//如果是动态SQL,则直接使用TextSqlNode类型,并将isDynamic标识置为true contents.add(textSqlNode); isDynamic = true; } else {//不是动态sql,则创建StaticTextSqlNode对象,表示静态SQL contents.add(new StaticTextSqlNode(data)); } } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { //其他类型的节点,由不同的节点处理器来对应处理成本成不同的SqlNode类型 String nodeName = child.getNode().getNodeName(); NodeHandler handler = nodeHandlerMap.get(nodeName); if (handler == null) { throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement."); } handler.handleNode(child, contents); isDynamic = true; } } //用contents构建MixedSqlNode对象 return new MixedSqlNode(contents); }
上述过程中,我们主要关注静态SQL的解析过程,对于动态SQL的解析将在之后介绍。
得到MixedSqlNode后,静态的SQL会创建出RawSqlSource对象。
看一下RawSqlSource:
public class RawSqlSource implements SqlSource { //内部封装的sqlSource对象,getBoundSql方法会委托给这个对象 private final SqlSource sqlSource; public RawSqlSource(Configuration configuration, SqlNode rootSqlNode, Class<?> parameterType) { this(configuration, getSql(configuration, rootSqlNode), parameterType); } public RawSqlSource(Configuration configuration, String sql, Class<?> parameterType) { //创建sqlSourceBuilder SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration); Class<?> clazz = parameterType == null ? Object.class : parameterType; //解析sql,创建StaticSqlSource对象 sqlSource = sqlSourceParser.parse(sql, clazz, new HashMap<String, Object>()); } //获取sql语句 private static String getSql(Configuration configuration, SqlNode rootSqlNode) { DynamicContext context = new DynamicContext(configuration, null); //这里的rootSqlNode就是之前得到的MixedSqlNode,它会遍历内部的SqlNode,逐个调用sqlNode的apply方法。StaticTextSqlNode会直接context.appendSql方法 rootSqlNode.apply(context); return context.getSql(); } @Override public BoundSql getBoundSql(Object parameterObject) { return sqlSource.getBoundSql(parameterObject); } }
代码相对简单,主要的步骤就是(1)通过SqlNode获得原始SQL语句;(2)创建SqlSourceBuilder对象,解析SQL语句,并创建StaticSqlSource对象;(3)将getBoundSql方法委托给内部的staticSqlSource对象。
其中比较关键的一步是解析原始SQL语句,并创建StaticSqlSource对象。因此我们继续看SqlSourceBuilder对象。
public class SqlSourceBuilder extends BaseBuilder { private static final String parameterProperties = "javaType,jdbcType,mode,numericScale,resultMap,typeHandler,jdbcTypeName"; public SqlSourceBuilder(Configuration configuration) { super(configuration); } public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) { //创建TokenHandler,用来将原始Sql中的'#{}' 解析成'?' ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters); GenericTokenParser parser = new GenericTokenParser("#{", "}", handler); //解析原始sql String sql = parser.parse(originalSql); //创建出StaticSqlSource对象 return new StaticSqlSource(configuration, sql, handler.getParameterMappings()); } private static class ParameterMappingTokenHandler extends BaseBuilder implements TokenHandler { private List<ParameterMapping> parameterMappings = new ArrayList<ParameterMapping>(); private Class<?> parameterType; private MetaObject metaParameters; public ParameterMappingTokenHandler(Configuration configuration, Class<?> parameterType, Map<String, Object> additionalParameters) { super(configuration); this.parameterType = parameterType; this.metaParameters = configuration.newMetaObject(additionalParameters); } public List<ParameterMapping> getParameterMappings() { return parameterMappings; } @Override public String handleToken(String content) { //解析'#{}'中的参数,创建ParameterMapping对象 parameterMappings.add(buildParameterMapping(content)); //将'#{}'替换成'?' return "?"; } private ParameterMapping buildParameterMapping(String content) { Map<String, String> propertiesMap = parseParameterMapping(content); String property = propertiesMap.get("property"); Class<?> propertyType; if (metaParameters.hasGetter(property)) { // issue #448 get type from additional params propertyType = metaParameters.getGetterType(property); } else if (typeHandlerRegistry.hasTypeHandler(parameterType)) { propertyType = parameterType; } else if (JdbcType.CURSOR.name().equals(propertiesMap.get("jdbcType"))) { propertyType = java.sql.ResultSet.class; } else if (property == null || Map.class.isAssignableFrom(parameterType)) { propertyType = Object.class; } else { MetaClass metaClass = MetaClass.forClass(parameterType, configuration.getReflectorFactory()); if (metaClass.hasGetter(property)) { propertyType = metaClass.getGetterType(property); } else { propertyType = Object.class; } } ParameterMapping.Builder builder = new ParameterMapping.Builder(configuration, property, propertyType); Class<?> javaType = propertyType; String typeHandlerAlias = null; for (Map.Entry<String, String> entry : propertiesMap.entrySet()) { String name = entry.getKey(); String value = entry.getValue(); if ("javaType".equals(name)) { javaType = resolveClass(value); builder.javaType(javaType); } else if ("jdbcType".equals(name)) { builder.jdbcType(resolveJdbcType(value)); } else if ("mode".equals(name)) { builder.mode(resolveParameterMode(value)); } else if ("numericScale".equals(name)) { builder.numericScale(Integer.valueOf(value)); } else if ("resultMap".equals(name)) { builder.resultMapId(value); } else if ("typeHandler".equals(name)) { typeHandlerAlias = value; } else if ("jdbcTypeName".equals(name)) { builder.jdbcTypeName(value); } else if ("property".equals(name)) { // Do Nothing } else if ("expression".equals(name)) { throw new BuilderException("Expression based parameters are not supported yet"); } else { throw new BuilderException("An invalid property '" + name + "' was found in mapping #{" + content + "}. Valid properties are " + parameterProperties); } } if (typeHandlerAlias != null) { builder.typeHandler(resolveTypeHandler(javaType, typeHandlerAlias)); } return builder.build(); } private Map<String, String> parseParameterMapping(String content) { try { return new ParameterExpression(content); } catch (BuilderException ex) { throw ex; } catch (Exception ex) { throw new BuilderException("Parsing error was found in mapping #{" + content + "}. Check syntax #{property|(expression), var1=value1, var2=value2, ...} ", ex); } } } }
parse方法主要分为以下几步:
(1)创建了ParameterMappingTokenHandler对象
(2)将ParameterMappingTokenHandler对象传入GenericTokenParser的构造函数中,创建GenericTokenParser对象
(3)通过GenericTokenParser对象解析原始SQL,这个过程中会将#{}替换成?,并将#{}中的参数,解析形成ParamterMapping对象
(4)用得到的SQL和ParamterMapping对象创建StaticSqlSource对象。
解析完成后回到一开始的XMLMapperBuilder,它会在资源添加到已加载的列表中,并bindMapperForNamespace方法中为创建的MappedStatement添加命名空间。